Method for producing semiconductor piece, circuit board and electronic device including semiconductor piece, and method for designing etching condition

ABSTRACT

A method for producing a semiconductor piece includes forming a first groove portion of a front-surface-side groove by anisotropic dry etching from a front surface of a substrate, forming a second groove portion of the front-surface-side groove, the second groove portion being located below and in communication with the first groove portion and having a width wider than a width of the first groove portion, and thinning the substrate from a back surface of the substrate up to the second groove portion. The second groove portion is formed by changing an etching condition of the anisotropic dry etching during the formation of the front-surface-side groove so that the width of the second groove portion is wider than the width of the first groove portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2014-182116 filed Sep. 8, 2014 andJapanese Patent Application No. 2015-106148 filed May 26, 2015.

BACKGROUND

(i) Technical Field

The present invention relates to a method for producing a semiconductorpiece, a circuit board and an electronic device that include asemiconductor piece, and a method for designing an etching condition.

(ii) Related Art

An example of a method for increasing the number of semiconductor piecesthat can be obtained from a single substrate is a method includingforming a front-surface-side groove by etching from a front surface of asubstrate, and thinning the substrate from a back surface of thesubstrate up to the front-surface-side groove to divide the substrateinto plural semiconductor pieces.

SUMMARY

According to an aspect of the invention, there is provided a method forproducing a semiconductor piece, the method including forming a firstgroove portion of a front-surface-side groove by anisotropic dry etchingfrom a front surface of a substrate, forming a second groove portion ofthe front-surface-side groove, the second groove portion being locatedbelow and in communication with the first groove portion and having awidth wider than a width of the first groove portion, and thinning thesubstrate from a back surface of the substrate up to the second grooveportion, in which the second groove portion is formed by changing anetching condition of the anisotropic dry etching during the formation ofthe front-surface-side groove so that the width of the second grooveportion is wider than the width of the first groove portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a flowchart showing an example of steps of producingsemiconductor pieces according to an exemplary embodiment of theinvention;

FIGS. 2A to 2D are schematic cross-sectional views of a semiconductorsubstrate in steps of producing semiconductor pieces according to anexemplary embodiment of the invention;

FIGS. 3A to 3F are schematic cross-sectional views of a semiconductorsubstrate in steps of producing semiconductor pieces according to anexemplary embodiment of the invention;

FIG. 4 is a schematic plan view of a semiconductor substrate (wafer)when the formation of circuit is completed;

FIG. 5 is a schematic perspective view of a semiconductor chip accordingto a first exemplary embodiment of the invention;

FIGS. 6A and 6B are views illustrating adhesion of a semiconductor chipto a printed wiring board when the semiconductor chip has a verticalshape;

FIGS. 7A to 7D are views illustrating the comparison between adhesion ofa semiconductor chip according to an exemplary embodiment to a printedwiring board and adhesion of a vertical-shaped semiconductor chip to aprinted wiring board;

FIGS. 8A to 8D are cross-sectional views illustrating typical structuresof fine grooves according to exemplary embodiments of the invention;

FIG. 9 is a schematic perspective view of a semiconductor chip accordingto another exemplary embodiment of the invention;

FIGS. 10A and 10B are cross-sectional views illustrating structures offine grooves according to other exemplary embodiments of the invention;

FIG. 11 is a view illustrating a state in which an adhesive layerreaches inside of a fine groove in a back-grinding step;

FIG. 12 is a cross-sectional view illustrating a residue of an adhesivelayer that remains during the detachment of a dicing tape from a frontsurface of a substrate;

FIG. 13 is a flowchart showing a first production method for producing afine groove according to an exemplary embodiment of the invention; and

FIGS. 14A and 14B are schematic cross-sectional views illustrating stepsof producing a flask-shaped fine groove in the first production methodaccording to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

A method for producing a semiconductor piece according to an exemplaryembodiment of the invention is applicable to, for example, a method forproducing individual semiconductor pieces (semiconductor chips) bydividing (into pieces) a substrate-like member, such as a semiconductorwafer, on which plural semiconductor elements are formed. Examples ofthe semiconductor elements formed on the substrate include, but are notparticularly limited to, light-emitting elements, active elements, andpassive elements. For example, the method according to an exemplaryembodiment of the invention is applicable to a method for isolatingsemiconductor pieces including light-emitting elements from a substrate.The light-emitting elements may each be, for example, a surface-emittingsemiconductor laser, a light-emitting diode, or a light-emittingthyristor. One semiconductor piece may include a single light-emittingelement or may include plural light-emitting elements arranged in theform of an array. Furthermore, one semiconductor piece may include adriving circuit that drives such a single or plural light-emittingelements. Examples of the substrate include, but are not limited to,substrates composed of silicon, SiC, a compound semiconductor, orsapphire. The substrate may be composed of another material as long asthe substrate is a substrate including at least a semiconductor(hereinafter, collectively referred to as a “semiconductor substrate”).An example of the substrate is a group III-V compound semiconductorsubstrate, such as a GaAs substrate, on which a light-emitting elementsuch as a surface-emitting semiconductor laser or a light-emitting diodeis formed.

In the description below, a method including forming plurallight-emitting elements on a semiconductor substrate and isolatingindividual semiconductor pieces (semiconductor chips) from thesemiconductor substrate will be described with reference to thedrawings. It is to be noted that the scale and the shape in the drawingsare emphasized in order to easily understand features of the inventionand are not necessarily the same as the scale and the shape of actualdevices.

FIG. 1 is a flowchart showing an example of steps of producingsemiconductor pieces according to an exemplary embodiment of theinvention. As shown in FIG. 1, a method for producing semiconductorpieces of the present exemplary embodiment includes a step of forminglight-emitting elements (S100), a step of forming a resist pattern(S102), a step of forming fine grooves on a front surface of asemiconductor substrate (S104), a step of stripping the resist pattern(S106), a step of reversing the semiconductor substrate and attaching adicing tape to the front surface of the substrate (S108), a step ofthinning the semiconductor substrate by grinding a back surface of thesemiconductor substrate by machining or the like (S110), a step ofirradiating the dicing tape with ultraviolet (UV) light and attaching anexpanding tape to the back surface of the semiconductor substrate(S112), a step of detaching the dicing tape and irradiating theexpanding tape with ultraviolet light (S114), and a step of picking asemiconductor piece (semiconductor chip) with a collet and die-mountingthe semiconductor chip on a printed circuit board or the like (S116).Cross-sectional views of a semiconductor substrate illustrated in FIGS.2A to 2D and FIGS. 3A to 3F correspond to the steps S100 to S116.

In the step of forming light-emitting elements (S100), as illustrated inFIG. 2A, plural light-emitting elements 100 are formed on a frontsurface of a semiconductor substrate W composed of GaAs or the like.Each of the light-emitting elements 100 is, for example, asurface-emitting semiconductor laser, a light-emitting diode, or alight-emitting thyristor. In the drawings, one region is illustrated asa light-emitting element 100. However, a single light-emitting element100 exemplifies an element included in a divided single semiconductorpiece. It is to be noted that one region of a light-emitting element 100may include a single light-emitting element, or plural light-emittingelements and other circuit elements. Furthermore, in order to easilyunderstand the explanation, the light-emitting elements 100 areillustrated with an emphasis so as to protrude from the front surface ofthe substrate. However, the light-emitting elements 100 may be formed soas to be substantially flush with the front surface of the substrate.

FIG. 4 is a plan view illustrating an example of a semiconductorsubstrate W when the step of forming light-emitting elements iscompleted. For the sake of convenience, only light-emitting elements 100in a central portion are illustrated as examples in the figure. Thelight-emitting elements 100 are formed on a front surface of thesemiconductor substrate W in the form of an array in the row directionand in the column direction. A planar region of one light-emittingelement 100 substantially has a rectangular shape. The light-emittingelements 100 are separated from each other in a grid-like manner bycutting regions 120 that are specified by scribe lines or the likearranged at uniform intervals S.

After the formation of the light-emitting elements is completed, aresist pattern is formed on the front surface of the semiconductorsubstrate W (S102). As illustrated in FIG. 2B, a resist pattern 130 isprocessed so as to expose the cutting regions 120 specified by scribelines or the like on the front surface of the semiconductor substrate W.The resist pattern 130 is processed by photolithography.

Next, fine grooves are formed on the front surface of the semiconductorsubstrate W (S104). As illustrated in FIG. 2C, very small grooves (forthe sake of convenience, hereinafter referred to as “fine grooves” or“front-surface-side groove”) 140 having a uniform depth are formed onthe front surface of the semiconductor substrate W using the resistpattern 130 as a mask. These fine grooves may be formed by anisotropicdry etching, for example, anisotropic plasma etching (reactive ionetching). The fine grooves 140 of the present exemplary embodiment eachhave a shape in which a width of the substrate-back-surface side islarger than a width of the substrate-front-surface side. An example ofthe fine groove 140 is illustrated in FIG. 2D in an enlarged manner. Inthis fine groove 140, a width Sb of the substrate-back-surface side islarger than a width Sa of the substrate-front-surface side (Sb>Sa). Thefine groove 140 has an inverted mesa shape in which side surfaces of thegroove linearly incline. Other shapes of the fine groove 140 and detailsof a method for producing the fine groove 140 will be described below.

By using anisotropic dry etching, the fine groove 140 is formed so as tohave a narrow width and a large depth compared with the case where thefine groove 140 is formed using a dicing blade having a small thickness.Furthermore, the effects of vibrations, a stress, etc. on thelight-emitting elements 100 near the fine groove are suppressed comparedwith the case where a dicing blade is used. The width Sa of thesubstrate-front-surface side of the fine groove 140 is substantially thesame as the width of an opening formed in the resist pattern 130. Thewidth Sa is, for example, several micrometers to ten-odd micrometers.The depth of the fine groove 140 is, for example, about 10 to 100 μm.The fine groove 140 is formed so that at least the depth thereof islarger than a depth in which a functional element such as alight-emitting element is formed. In the case where the fine groove 140is formed using a typical dicing blade, the interval S of the cuttingregions 120 is determined as a total of a groove width of the dicingblade and a margin width in which the amount of chipping is consideredand thus is large, namely, about 40 to 60 μm. In contrast, in the casewhere the fine groove 140 is formed by a semiconductor process, not onlythe groove width is narrow, but also a margin width for cutting isnarrow compared with the case where the fine groove 140 is formed usinga dicing blade. In other words, since the interval S of the cuttingregions 120 is small, the number of semiconductor pieces obtained isincreased by arranging light-emitting elements on a wafer at a highdensity. In the present exemplary embodiment, the term “front-surfaceside” refers to a side of a surface on which functional elements such aslight-emitting elements are formed, and the term “back-surface side”refers to a side of the opposite surface.

Next, the resist pattern is stripped (S106). As illustrated in FIG. 2D,after the resist pattern 130 is stripped from the front surface of thesemiconductor substrate W, the fine grooves 140 formed along the cuttingregions 120 are exposed on the front surface.

Next, the semiconductor substrate W is reversed, and anultraviolet-curable dicing tape is attached to the front surface of thesubstrate (S108). As illustrated in FIG. 3A, a dicing tape 160 having anadhesive layer is attached to the light-emitting element side. Thus, thefront surface of the substrate is protected.

Next, the back surface of the substrate is ground by back-grinding(machining), thereby thinning the substrate (S110). The thinning byback-grinding is performed until the fine grooves 140 are exposed asillustrated in FIG. 3B. The back-grinding is performed by, for example,moving a rotating grindstone 170 in a horizontal direction or in avertical direction. Thus, the thickness of the substrate becomesuniform. The step of thinning the substrate may be performed by chemicalmechanical polishing (CMP) in addition to back-grinding.

Next, the semiconductor substrate W is reversed, the dicing tape 160attached to the front surface of the substrate is irradiated withultraviolet (UV) light, and an expanding tape is attached to the backsurface of the substrate (S112). As illustrated in FIG. 3C, the dicingtape 160 is irradiated with ultraviolet light 180 to cure the adhesivelayer thereof, and an expanding tape 190 is attached to the back surfaceof the semiconductor substrate W. The order of the irradiation with theultraviolet light 180 and the attachment of the expanding tape 190 isnot limited. Either the irradiation with the ultraviolet light 180 orthe attachment of the expanding tape 190 may be performed earlier.

Next, the dicing tape is detached, and the expanding tape is irradiatedwith ultraviolet light (S114). As illustrated in FIG. 3D, the dicingtape 160 is detached from the front surface of the semiconductorsubstrate W. The expanding tape 190 includes a base having a stretchingproperty. The expanding tape is stretched so as to facilitate picking upof semiconductor pieces divided after dicing, thus expanding theinterval of the light-emitting elements.

The expanding tape 190 is irradiated with ultraviolet light 200 to curean adhesive layer thereof. Subsequently, picking and die-mounting of adivided semiconductor piece are performed (S116). As illustrated inFIGS. 3E and 3F, a semiconductor piece 210 picked from the expandingtape 190 with a collet is caused to adhere onto a circuit board 230 withan adhesive 220 therebetween, the adhesive 220 being, for example, aconductive paste such as solder.

Next, a description will be made of mounting (die-mounting) of asemiconductor chip divided into a piece in the steps of the presentexemplary embodiment. FIG. 5 is a schematic perspective view of asemiconductor chip divided into a piece through the formation of theinverted mesa-shaped fine grooves illustrated in FIGS. 2C to 3E. Asemiconductor chip 210 is substantially constituted by a hexahedronincluding a rectangular front surface 300, a rectangular back surface310, and four side surfaces 320 connecting the front surface 300 and theback surface 310. The four side surfaces 320 are etched surfaces formedwhen the fine grooves 140 are formed. The back surface 310 is a groundsurface formed by back-grinding. In the case where the fine grooves 140form side surfaces that uniformly and linearly incline from the width Saof the substrate-front-surface side to the width Sb of thesubstrate-back-surface side as illustrated in FIG. 2D, the side surfaces320 of the semiconductor chip 210 linearly incline so that the sizedecreases from the front surface 300 to the back surface 310. That is,the area of the front surface 300 is represented by “width Xa×width Ya”,and the area of the back surface 310 is represented by “width Xb×widthYb” (where Xa>Xb, and Ya>Yb). The area of the back surface 310 issmaller than the area of the front surface 300.

Next, mounting of the semiconductor chip on a circuit board will bedescribed. First, as a comparative embodiment, mounting of asubstantially rectangular parallelepiped semiconductor chip whose sidesurfaces have a vertical shape will be described. FIG. 6A is a schematicside view of a semiconductor chip mounted on a circuit board, and FIG.6B is a top view of the semiconductor chip mounted on the circuit board.A rectangular parallelepiped semiconductor chip 10 includes a frontsurface 12, a back surface 16, and four side surfaces 14 connecting thefront surface 12 and the back surface 16. Unlike the semiconductor chip210 of the present exemplary embodiment, in this semiconductor chip 10,the side surfaces 14 are orthogonal to the front surface 12 and the backsurface 16. That is, the area of the front surface 12 is substantiallythe same as the area of the back surface 16.

The back surface 16 of the semiconductor chip 10 is bonded with anadhesive 22 applied onto a surface of a circuit board 20. When thesemiconductor chip 10 is mounted on the circuit board 20, thesemiconductor chip 10 is pressed with a certain amount of force onto thecircuit board 20. Consequently, part of the viscous adhesive 22protrudes laterally from the back surface 16. In this case, a distanceof the adhesive 22 protruding from a side surface of the semiconductorchip in a direction perpendicular to the side surface is represented bya protrusion distance d. An actual plane occupancy area necessary forsurface mounting of the semiconductor chip 10 is not equal to the sizeof the back surface 16 but to the size when the protrusion distance d ofthe adhesive 22 is taken into account. Specifically, the plane occupancyarea is increased by an amount corresponding to the protrusion distanced. In order to reduce costs, it is desirable to reduce chip size.However, even if chip size is reduced, when the protrusion distance d ofthe adhesive 22 is large, the plane occupancy area cannot be decreased.Therefore, the effect due to the reduction in chip size cannot besufficiently realized. For example, in a device in which pluralsemiconductor chips are arranged on a circuit board either linearly orin a zigzag manner, the reduction in size and the reduction in thicknessof the device are not sufficiently achieved unless the plane occupancyarea is reduced.

FIGS. 7A and 7B are respectively a side view and a front view when thevertical-shaped semiconductor chip 10 illustrated in FIGS. 6A and 6B ismounted. FIGS. 7C and 7D are respectively a side view and a front viewwhen the inverted mesa-shaped semiconductor chip 210 illustrated in FIG.5 is mounted. As illustrated in FIGS. 7A and 7B, when thevertical-shaped semiconductor chip 10 is mounted, the plane occupancyarea is the area determined by adding the protrusion distance d to thearea of the front surface or the back surface of the semiconductor chip10. When the semiconductor chip 210 of the present exemplary embodimentis mounted on a circuit board 230, an adhesive 220 protrudes from theback surface 310 in four directions. However, since the side surfaces320 incline inwardly from the front surface 300 to the back surface 310,the protruding adhesive 220 is hidden by the inclination of the sidesurfaces 320, when the semiconductor chip 210 is viewed from the top.That is, even when the adhesive 220 protrudes from the back surface 310of the semiconductor chip 210, the protrusion distance d of the adhesive220 does not increase the plane occupancy area as it is because the sidesurfaces 320 incline. The plane occupancy area of the semiconductor chip210 is dominantly determined by the front surface 300 having a largearea. However, even if the protrusion distance d is generated, theprotrusion distance d is reduced by the differences Xa−Xb and Ya−Yb inwidth between the front surface 300 and the back surface 310. If thedifferences Xa−Xb and Ya−Yb in width between the front surface 300 andthe back surface 310 are each larger than the protrusion distance d, theplane occupancy area is not affected by the protrusion distance d.Furthermore, when a base angle θ of the back surface 310 is an obtuseangle, a junction area between the adhesive 220 and the side surface 320increases. Therefore, the protrusion distance d is smaller than that inthe case where the vertical-shaped semiconductor chip 10 is mounted.

Next, various structural examples of the fine groove that are applicableto the present exemplary embodiment will be described. The fine groove140 according to the present exemplary embodiment is processed so that awidth of the bottom thereof is broadened in a direction parallel to asurface of the substrate. FIGS. 8A to 8D illustrate typical structuralexamples of the fine groove. A fine groove 500 illustrated in FIG. 8Aincludes a first groove portion 510 having a depth D1 and having linearside surfaces that form a substantially uniform width Sa1, and a secondgroove portion 520 connected below the first groove portion 510, havinga depth D2, and having spherical side and reverse surfaces. A width Sa2of the second groove portion 520 is an inner diameter between facingsidewalls in a direction parallel to the surface of the substrate. Therelationship Sa2>Sa1 is satisfied. In the example illustrated in FIG.8A, the second groove portion 520 has a maximum of the width Sa2 in thevicinity of the center thereof.

A fine groove 500A illustrated in FIG. 8B includes a first grooveportion 510 having a depth D1 and having linear side surfaces that forma substantially uniform width Sa1, and a rectangular second grooveportion 530 connected below the first groove portion 510, having a depthD2, and having substantially linear side surfaces. The second grooveportion 530 has a structure in which the spherical side and reversesurfaces of the second groove portion 520 illustrated in FIG. 8A arechanged in a linear manner. A width Sa2 of the second groove portion 530is a distance between facing sidewalls in a direction parallel to thesurface of the substrate. This distance is substantially uniform(Sa2>Sa1). It is to be noted that the shape of the second groove portionillustrated in the figures is only illustrative. The shape of the secondgroove portion is not particularly limited as long as the second grooveportion has a width wider than the width Sa1 of the first grooveportion. For example, the shape of the second groove portion may be anintermediate shape between the second groove portion 520 illustrated inFIG. 8A and the second groove portion 530 illustrated in FIG. 8B.Specifically, the second groove portion may have an ellipticalcross-sectional shape. Furthermore, in other words, the shape of thesecond groove portion is not particularly limited as long as the secondgroove portion has a space having a width wider than a width of thegroove in a boundary portion with the first groove portion (i.e., widthof the groove at a depth of D1).

A fine groove 500B illustrated in FIG. 8C includes a first grooveportion 510 having a depth D1 and having side surfaces that form asubstantially uniform width Sa1, and a reverse-tapered second grooveportion 540 connected below the first groove portion 510 and having adepth D2. The side surfaces of the second groove portion 540 areinclined so that the width gradually increases toward the bottom. Awidth Sa2 of the second groove portion 540 is a distance between facingside surfaces in a direction parallel to the surface of the substrate.The second groove portion 540 has a maximum of the distance in thevicinity of the lowest portion (in the vicinity of the lower end)thereof. In FIG. 8C, side surfaces of the first groove portion 510 mayincline so that the width gradually increases toward the bottom as longas the angle formed by the side surfaces is different from an angle ofinclination of the side surfaces of the second groove portion 540.

A fine groove 500C illustrated in FIG. 8D has a shape in which the widthgradually increases from an opening width Sa1 of a surface of thesubstrate to a width Sa2 in the vicinity of a lowest portion. That is,the fine groove 500C is constituted by a reverse-tapered groove having adepth D2. The fine groove 500C has a structure in which the depth D1 ofthe first groove portion 510 illustrated in FIG. 8C is reducedinfinitely. The semiconductor chip 210 illustrated in FIG. 5 is obtainedby dividing a substrate on which the fine groove 500C illustrated inFIG. 8D is formed. Unlike the shapes illustrated in FIGS. 8A to 8C, theshape illustrated in FIG. 8D is not a shape in which the angle of a sidesurface changes at the boundary between the first groove portion and thesecond groove portion. Rather, the shape illustrated in FIG. 8D is ashape in which the groove width increases toward a lower portion, whencomparing an upper portion with a lower portion of the whole groove.Thus, the shape illustrated in FIG. 8D includes a first groove portion(upper portion) and a second groove portion (lower portion) having awidth wider than a width of the first groove portion.

As illustrated in FIGS. 8A to 8C, the shape including the first grooveportion 510 that has a depth D1 and that includes linear side surfacesforming a substantially uniform width Sa1 easily suppresses chipping andcracking of corner portions of a semiconductor chip compared with thecompletely inverted mesa shape illustrated in FIG. 8D. FIG. 9 is aperspective view of a semiconductor chip 210A obtained by dividing asubstrate on which the fine groove 500B illustrated in FIG. 8C isformed. As illustrated in FIG. 9, side surfaces 320A thatperpendicularly extend from a front surface 300 are formed on the frontsurface 300 of the semiconductor chip 210A. The side surfaces 320Acorrespond to the first groove portion 510. As illustrated in FIG. 5, inthe case where the side surfaces 320 extend from the front surface 300at an acute angle, chipping and cracking of boundary portions betweenthe front surface 300 and the side surfaces 320 easily occur. Incontrast, the side surfaces 320A illustrated in FIG. 9 suppress chippingand cracking.

Next, the step of thinning a substrate by back-grinding will bedescribed. In back-grinding, the back surface of the substrate isground, and the substrate is processed to have a thickness such that thefine groove 140 is exposed. The thickness of the substrate may beselected so that the area of the back surface of the semiconductor chipis optimized in accordance with the shapes of the fine groovesillustrated in FIGS. 8A to 8D. In the case where the fine groove 500illustrated in FIG. 8A is formed, the grinding by back-grinding iscontrolled in a range in which the thickness of the substrate exceeds atleast the half of the depth D2 of the second groove portion 520 and doesnot reach the first groove portion 510. The back surface of thesemiconductor chip has a minimum area at a depth of D2/2. In the casewhere the fine groove 500A illustrated in FIG. 8B is formed, thegrinding is controlled so that the thickness of the substrate is withinthe range of the depth D2 of the second groove portion 530. The area ofthe back surface of the semiconductor chip is uniform within this range.In the cases where the fine grooves 500B and 500C illustrated in FIGS.8C and 8D are formed, respectively, the grinding is controlled so thatthe thickness of the substrate is within the range of the depth D2 ofthe second groove portion 540 or 550. The back surface of thesemiconductor chip has a minimum area when the thickness of thesubstrate is in the vicinity of the bottom of the second groove portion540 or 550.

Next, a description will be made of a fine groove that is effective forsuppressing a residue of an adhesive layer of a dicing tape, the residueremaining during the detachment of the dicing tape. Regarding the shapeof the first groove portion of the fine groove, the adhesive layer doesnot easily remain in a groove having a vertical shape illustrated in anyof FIGS. 8A to 8C compared with a shape (reverse-tapered shape) in whichthe width gradually increases from the front surface to the back surfaceof the substrate as illustrated in FIG. 8D. The reason for this is asfollows. In the case of a groove having a reverse-tapered shape,ultraviolet light is not easily applied to an adhesive layer that hasreached inside of the groove, and thus the adhesive layer is not easilycured. Even if the adhesive layer is cured, during detachment of adicing tape, a stress is easily applied to a root portion of theadhesive layer that has reached inside of the groove as compared withthe case where a groove has a vertical shape. Consequently, the adhesivelayer is torn easily. Furthermore, regarding the shape of the firstgroove portion, the adhesive layer does not easily remain in a groovehaving a shape (forward-tapered shape) in which the width graduallydecreases from the front surface to the back surface of the substratecompared with the vertical shapes illustrated in FIGS. 8A to 8C. FIG.10A illustrates an example of a fine groove including a first grooveportion having a forward-tapered shape and a second groove portionhaving a reverse-tapered shape. As illustrated in FIG. 10A, a finegroove 500D has a shape in which the vertical-shaped first grooveportion 510 illustrated in FIG. 8C is changed to a groove portion 560having a forward-tapered shape. The fine groove 500D has facing sidesurfaces that incline from an opening width Sa1 of the front surface ofthe substrate to a width Sa3 at a depth D1 in a forward direction, andfacing surfaces that incline from the width Sa3 to a width Sa2 at thebottom in a reverse direction. In FIG. 10A, the relationship Sa2>Sa1>Sa3is satisfied. However, regarding the relationship between Sa1 and Sa3,either Sa1 or Sa3 may be larger than the other.

Next, a residue of the adhesive layer that remains during the detachmentof the dicing tape will be described with reference to FIGS. 11 and 12.It is assumed that fine grooves 30 each having a vertical shapeincluding linear side surfaces that form a uniform width Sa1 are formedon a front surface of a substrate. As illustrated in FIG. 11, during thestep of back-grinding, a vibration B and a cutting pressure P areapplied to an adhesive layer 164 through inner walls of the fine grooves30 as a result of the rotation of a grindstone 170, relative movementbetween the grindstone 170 and the semiconductor substrate W, etc. Whenthe semiconductor substrate W is pressed by the cutting pressure P in aY direction, the viscous adhesive layer 164 flows and reaches inside ofa fine groove 30. In addition, the vibration B is transmitted to thevicinity of the fine groove 30, thereby promoting the flow of theadhesive layer 164.

After the grinding with the grindstone 170 is completed, an expandingtape 190 is attached to a back surface of the substrate. The dicing tape160 is irradiated with ultraviolet light 180. The adhesive layer 164that has been irradiated with ultraviolet light is cured, and adhesionof the adhesive layer 164 is lost. As illustrated in FIG. 12, the dicingtape 160 is detached from the front surface of the substrate. Theexpanding tape 190 includes a tape base 192 and an adhesive layer 194stacked on the tape base 192. The expanding tape 190 holds the cutsemiconductor pieces with the adhesive layer 194.

In this case, an adhesive layer 164 a that has reached inside of thefine groove 30 tends to be uncured because part of the adhesive layer164 a is not sufficiently irradiated with ultraviolet light. The uncuredadhesive layer 164 has adhesiveness. Accordingly, when the adhesivelayer 164 is detached from the front surface of the substrate, theuncured adhesive layer 164 a may cut, and consequently, the adhesivelayer 164 a may remain in the fine groove 30 or may adhere to the frontsurface of the substrate again and remain. Even if the adhesive layer164 a is in a cured state, since the adhesive layer 164 a has reacheddeeply the narrow fine groove, the adhesive layer 164 a may be torn by astress during the detachment and remain. If a residual adhesive layer164 b adheres to a surface of a light-emitting element again, the amountof light emitted from the light-emitting element decreases, and thelight-emitting element is considered to be a defective element,resulting in a decrease in the yield. Similarly, in the case of asemiconductor chip other than a light-emitting element, other adverseeffects are assumed. For example, due to the presence of the residue ofthe adhesive layer 164 b, the semiconductor chip may be determined to bedefective in an appearance inspection of the chip or the like. For thesereasons, residues of the adhesive layers 164 a and 164 b that remain onthe front surface of the substrate during the detachment of the dicingtape are not desirable.

In order to suppress such a residue of an adhesive layer that remainsduring the detachment of the dicing tape, the shape of the first grooveportion of the fine groove may be a forward-tapered shape in which thewidth gradually decreases from the front surface to the back surface ofthe substrate, as illustrated in FIG. 10A. This is because, in the caseof the forward-tapered shape, the adhesive layer is easily irradiatedwith ultraviolet light and the cured adhesive layer is easily removedfrom the fine groove compared with the cases of the vertical shapes(illustrated in FIGS. 8A to 8C) and the inverted mesa shape (illustratedin FIG. 8D). However, the use of a fine groove including a first grooveportion having a vertical shape or an inverted mesa shape is noteliminated. The generation of a residue of an adhesive layer in finegrooves depends on the conditions such as the width of each of the finegrooves, the pitch of the fine grooves, the viscosity of the adhesivelayer, etc. Even when the first groove portion has a vertical shape oran inverted mesa shape, fine grooves having any of these shapes may beused as long as the problem of the residue of the adhesive layer doesnot occur in practical use.

Next, a shape that is more effective for suppressing the residue of anadhesive layer than the groove shape illustrated in FIG. 10A will bedescribed. FIG. 10B illustrates an example of a shape that is moreeffective for suppressing the residue of an adhesive layer than thegroove shape illustrated in FIG. 10A. In FIG. 10B, an edge asillustrated in FIG. 10A is not provided between a forward-tapered shapeand a reverse-tapered shape, and an angle of a sidewall of the groovegradually changes from the forward-tapered shape to the reverse-taperedshape. According to this shape, even if an adhesive layer reaches adepth of the reverse-tapered shape, the adhesive layer does not easilyremain in the groove compared with the groove shape illustrated in FIG.10A because catching of the adhesive layer by an edge does not occurduring the detachment. As described below, this groove shape is formedby changing an etching strength of anisotropic dry etching not rapidlybut by such a strength difference that does not form an edge. Similarly,in the shape illustrated in FIG. 8C, the fine groove may be formed sothat an edge is not provided between the vertical shape and thereverse-tapered shape, and an angle of a sidewall of the groovegradually changes from the vertical shape to the reverse-tapered shape.According to this shape, even if an adhesive layer reaches a depth ofthe reverse-tapered shape, the adhesive layer does not easily remain inthe groove compared with the groove shape illustrated in FIG. 8C becausecatching of the adhesive layer by an edge does not occur during thedetachment.

The fine grooves 500, 500A, 500B, and 500C illustrated in FIGS. 8A to 8Dand the fine grooves 500D illustrated in FIGS. 10A and 10B,respectively, may be axisymmetric or may not be axisymmetric withrespect to a center line orthogonal to the substrate. In addition, FIGS.8A to 8D and FIGS. 10A and 10B are drawn by straight lines or curvedlines in order to clearly explain the features of the fine grooves. Itis to be noted that a side surface of a fine groove that is actuallyformed may include steps or irregularities, and that the corner does notnecessarily have an exact edge shape but may be formed by a curved line.Furthermore, FIGS. 8A to 8D and FIGS. 10A and 10B only illustrateexamples of the shape of a fine groove. The fine groove may have anothershape as long as a second groove portion having a width wider than afirst width is formed below and in communication with a first grooveportion. For example, in the shapes illustrated in FIGS. 10A and 10 b, agroove portion including side surfaces that are substantiallyperpendicular to the front surface of the substrate may be includedbetween the forward-tapered shape having a depth D1 and the invertedmesa shape having a depth D2. Alternatively, examples of the shape ofthe fine groove include shapes obtained by combining the shapes selectedfrom those illustrated in FIGS. 8A to 8D and FIGS. 10A and 10B, andshapes obtained by further modifying such a combined shape. The anglesof the forward-tapered shapes and the inverted mesa shapes illustratedin FIGS. 8A to 8D and FIGS. 10A and 10B are also only examples. Thedegree of inclination is not particularly limited as long as a sidesurface has an inclination with respect to a plane perpendicular to asubstrate surface.

Next, a method for producing a fine groove of the present exemplaryembodiment will be described. FIG. 13 is a flowchart showing a firstproduction method for producing a fine groove according to the presentexemplary embodiment. The method for producing a fine groove illustratedin any of FIGS. 8A to 8D and FIGS. 10A and 10B includes a step offorming a first groove portion having a width Sa1 by a first etching(S150), and subsequently, a step of forming a second groove portionhaving a width Sa2 larger than the width Sa1 below the first grooveportion by a second etching (S160). The second etching used in thismethod is an etching whose strength in a sidewall direction is higherthan that of the first etching. A description will be made of an exampleof a case where anisotropic dry etching is used as the first etching andisotropic dry etching is used as the second etching.

FIGS. 14A and 14B are schematic cross-sectional views illustrating stepsof producing the fine groove 500 illustrated in FIG. 8A. A photoresist700 is formed on a surface of a GaAs substrate W. The photoresist is,for example, an i-line resist having a viscosity of 100 cP. Thephotoresist is applied so as to have a thickness of about 8 μm. Anopening 710 is formed in the photoresist 700 by a knownphotolithographic process using, for example, an i-line stepper and adeveloper of 2.38% tetramethyl ammonium hydroxide (TMAH). The width ofthe opening 710 specifies a width Sa1 of a first groove portion.

A first groove portion 510 is formed on the surface of the substrate byanisotropic dry etching using the photoresist 700 as an etching mask.For example, inductively coupled plasma (ICP) is used in a reactive ionetching (RIE) apparatus. Regarding etching conditions, for example, thepower of the inductively coupled plasma (ICP) is 500 W, the bias poweris 50 W, the pressure is 3 Pa, the etchant gas includes Cl₂=150 sccm,BCl₃=50 sccm, and C₄F₈=20 sccm, and then etching time is 20 minutes. Asis publicly known, by adding a CF-based gas, a protective film 720 isformed on the sidewalls at the same time of the etching. Radicals andions are generated by plasma of reaction gases. The sidewalls of thegroove are attacked only by the radicals but are not etched because ofthe presence of the protective film 720. In contrast, the protectivefilm on the bottom is removed by the ions that are perpendicularlyincident. The portion from which the protective film is removed isetched by the radicals. Therefore, anisotropic etching is achieved.

Next, isotropic etching is performed by changing the etching conditions.For example, in the present exemplary embodiment, the supply of C₄F₈having a function of forming the sidewall protective film 720 isstopped. The power of the inductively coupled plasma (ICP) is 500 W, thebias power is 50 W, the pressure is 3 Pa, the etchant gas includesCl₂=150 sccm and BCl₃=50 sccm, and the etching time is 10 minutes. Sincethe supply of C₄F₈ is stopped, the sidewall protective film 720 is notformed. Thus, isotropic etching is achieved on the bottom of the firstgroove portion 510. As a result, a second groove portion 520 is formedbelow the first groove portion 510. The second groove portion 520 hasspherical side surface and reverse surface that extend from the widthSa1 of the first groove portion 510 in a lateral direction and in adownward direction. The etching conditions described above are merely anexample and may be appropriately changed in accordance with the width,the depth, the shape, and the like of the fine groove.

In order to form the shape illustrated in FIG. 8C, in the step offorming the second groove portion, the etching strength in a sidewalldirection is made to be lower than that in the case where the secondgroove portion illustrated in FIG. 8A is formed. The etching strength inthe sidewall direction may be changed by changing the etching conditionssuch as the output of an etching apparatus and an etchant gas used.Specifically, for example, the flow rate of C₄F₈, which is a gas forprotecting a sidewall, may be made to be lower than that during theformation of the first groove portion without completely stopping thesupply of C₄F₈. Alternatively, the flow rate of Cl₂ or the like, whichis a gas for etching, may be increased. Alternatively, these techniquesmay be combined. In other words, the first groove portion and the secondgroove portion may be formed by respectively changing the flow rates ofthe gas for protecting a sidewall and the gas for etching, the gasesbeing contained in the etchant gas, while supplying the two gases duringthe formation of both the first groove portion and the second grooveportion. These flow rates may be determined prior to the formation ofthe first groove portion. In this case, the first groove portion and thesecond groove portion are formed in a continuous etching step. In thecase where the first groove portion is formed so as to have a shape inwhich the width gradually decreases from the front surface to the backsurface of the substrate (forward-tapered shape) in order to suppressthe residue of an adhesive layer, the flow rates of C₄F₈ and Cl₂ and theoutput of an etching apparatus may be appropriately determined or theflow rates may be changed so that such a forward-tapered shape isformed. The shape illustrated in FIG. 8D is formed by omitting theformation of the first groove portion in FIG. 8C. Such etching isusually achieved as anisotropic dry etching. If the strength differencebetween before and after the change in the etching strength is large, anedge is formed between the first groove portion and the second grooveportion as in the shapes illustrated in FIGS. 8C and 10A, which mayresults in the residue of an adhesive layer. In order to suppress thisphenomenon, the degree of strength difference may be determined so thatan edge is not formed. Alternatively, the etching strength may begradually changed by determining the etching conditions in three or morestages so as to obtain such a strength difference. Consequently, a shapethat does not have an edge is formed, for example, as illustrated inFIG. 10B. Specifically, when the etching conditions are changed in threeor more stages, the change in the angle of a sidewall at the boundarybetween the first groove portion and the second groove portion becomesgentler.

Next, a description will be made of appropriate use of a case where agas for forming a protective film is stopped and a case where the flowrate of the gas is reduced without stopping the gas in the step offorming the second groove portion. In the case where the gas for forminga protective film is stopped, so-called isotropic dry etching isperformed. A groove formed in this case has a larger width than that inthe case where the second groove portion is formed by anisotropicetching. When a groove having a larger width is formed, the area of theback surface of a semiconductor chip is further reduced. However, in thecase where the second groove portion is formed by isotropic dry etching,a protective film that protects a sidewall is not newly formed on thefirst groove portion in addition to the second groove portion.Therefore, the protective film that has been formed on the sidewalls ofthe first groove portion is only etched by isotropic dry etching.Accordingly, if the protective film formed on the first groove portiondoes not have a sufficient thickness, during the formation of the secondgroove portion by isotropic dry etching, the protective film of thefirst groove portion may be perforated, and an unintended semiconductorlayer may also be etched. In particular, such a phenomenon easily occursin an inlet portion of a groove (in a range of a depth of about 10 μmfrom the inlet of a groove) because a fresh gas is easily supplied tothe inlet portion as compared with a bottom portion of the groove.

In general, elements such as light-emitting elements and activeelements, and peripheral functional portions such as wiring are formedon the front surface of a substrate, that is, also in the vicinity of aninlet portion of a groove. In order to suppress adverse effects on theseelements and the like, it is necessary that an unintended semiconductorlayer be not etched in the inlet portion of the first groove portion.Therefore, in the case where the protective film of the first grooveportion is perforated during the formation of the second groove portionby isotropic dry etching, the gas for forming a protective film is notcompletely stopped but the flow rate of the gas is only decreased duringthe formation of the second groove portion. Thus, etching conditions areselected so that the protective film of the first groove portion is notperforated, even though the groove width of the second groove portion issomewhat decreased.

Specifically, at the stage of design, if a hole is formed in an inletportion of the first groove portion, and an unintentional semiconductorlayer is etched during the formation of the second groove portion byisotropic dry etching, etching conditions in a mass production areselected in which a gas for forming a protective film is not completelystopped but the flow rate of the gas is only decreased during theformation of the second groove portion. In this manner, an unintentionalsemiconductor layer is prevented from being etched in the inlet portionof the first groove portion by designing the etching conditions so thatthe second groove portion is formed in a range in which a hole is notformed in the inlet portion of the first groove portion.

A method for producing a fine groove of the present exemplary embodimenthas been described. The following modifications may also be made. Thestructure of the front-surface-side groove is not particularly limitedas long as the front-surface-side groove includes at least a firstgroove portion and a second groove portion. Therefore, for example, athird groove portion and a fourth groove portion may be provided betweenthe first groove portion and the second groove portion or at a positioncloser to the back-surface side of the substrate than the second grooveportion. These groove portions may be formed by a third anisotropic orisotropic dry etching or a fourth anisotropic or isotropic dry etching.The second groove portion does not necessary have a width wider than thewidth of the lowest portion of the first groove portion. The reason forthis is as follows. In the case where, for example, the first grooveportion has a shape in which a width gradually decreases toward the backsurface of the substrate, by changing the conditions of dry etching sothat the degree of decrease in the width decreases, the area on theback-surface side becomes smaller than that in the case where thefront-surface-side groove is formed by single anisotropic dry etching.

Exemplary embodiments of the present invention have been described indetail. The exemplary embodiments, and functions and structuresdisclosed in the exemplary embodiments may be combined as long as theoperations and effects thereof are not inconsistent. The presentinvention is not limited to specific exemplary embodiments. Variousmodifications and changes may be made within the gist of the presentinvention described in the following claims.

What is claimed is:
 1. A method for producing a semiconductor piece, themethod comprising: forming a first groove portion of afront-surface-side groove by anisotropic dry etching from a frontsurface of a substrate; forming a second groove portion of thefront-surface-side groove, the second groove portion being located belowand in communication with the first groove portion and having a widthwider than a width of the first groove portion; and thinning thesubstrate from a back surface of the substrate up to the second grooveportion, wherein the second groove portion is formed by changing anetching condition of the anisotropic dry etching during the formation ofthe front-surface-side groove so that the width of the second grooveportion is wider than the width of the first groove portion.
 2. Themethod for producing a semiconductor piece according to claim 1, whereinthe method further comprises forming a protective film that protects agroove sidewall, wherein, in the anisotropic dry etching, a gas for theforming the protective film that protects the groove sidewall, the gasbeing contained in an etchant gas, flows at a first flow rate, andwherein the second groove portion is formed by changing the first flowrate of the gas to a second flow rate lower than the first flow rateduring the formation the front-surface-side groove.
 3. A method forproducing a semiconductor piece, the method comprising: forming a firstgroove portion by a first anisotropic dry etching from a front-surfaceside of a substrate; forming a second groove portion so as to be locatedbelow and in communication with the first groove portion; and thinningthe substrate from a back surface of the substrate up to the secondgroove portion, wherein the second groove portion is formed by changingan etching condition to a second anisotropic dry etching that can form awider groove than the first anisotropic dry etching can.
 4. A method forproducing a semiconductor piece, the method comprising: forming a firstgroove portion of a front-surface-side groove by dry etching from afront surface of a substrate; forming a second groove portion of thefront-surface-side groove, the second groove portion being located belowand in communication with the first groove portion and having a widthwider than a width of the first groove portion; attaching a holdingmember having an adhesive layer to the front surface on which thefront-surface-side groove is formed; thinning the substrate from a backsurface of the substrate up to the second groove portion; and after thethinning of the substrate, detaching the holding member from the frontsurface, wherein the second groove portion is formed by changing anetching condition of the dry etching during the formation of thefront-surface-side groove so that the width of the second groove portionis wider than the width of the first groove portion, and thefront-surface-side groove does not have an edge between the first grooveportion and the second groove portion and has a shape in which an angleof a sidewall of the front-surface-side groove gradually changes fromthe first groove portion to the second groove portion.
 5. A method fordesigning an etching condition used in the method for producing asemiconductor piece according to claim 1, the method comprising:selecting, as an etching condition for forming the second groove portionin a mass production, an etching condition in which a flow rate of a gasfor forming a protective film during the formation of the second grooveportion is made to be lower than that during the formation of the firstgroove portion without completely stopping the gas in the case where aprotective film formed during the formation of the first groove portionis perforated in an inlet portion of the first groove portion during theformation of the second groove portion by isotropic dry etching.